1. Field of the Invention
The present invention relates to a trouble diagnosing apparatus for passenger protective systems which are deployed at the time of a vehicle collision to protect passengers.
2. Description of the Related Art
Conventionally, a circuit, as shown in FIG. 7, for activating a passenger protective system such as an airbag system or a seatbelt retractor is well known. In this circuit, transistors 31, 34 are connected in series to the both ends of a squib 33. When the two transistors 31, 34 simultaneously turn on, a large current flows through the squib 33 so that the squib 33 is ignited.
To ensure the reliability of the passenger protective system, it is diagnosed, either periodically or at the time of an engine start, whether the transistors 31, 34 normally turn on in response to respective turn-on signals. In this diagnosis, the transistor 31 is determined to be normal if the potential of the squib 33 (the potential of the intermediate point M, hereinafter referred to as an intermediate point potential) becomes equal to a power source potential when only the transistor 31 is turned on. If not, the transistor 31 is determined to be abnormal. Also, the transistor 34 is determined to be normal if the intermediate point potential becomes equal to a ground potential when only the transistor 34 is turned on. If not, the transistor 34 is determined to be abnormal.
During this diagnosis, if a line L2 is being short-circuited to ground, or if a line L1 is being short-circuited to a power supply line, a large current is supplied to the squib 33, and thereby the squib 33 is erroneously ignited. To solve this problem, as shown in FIG. 7, resistors 32 and 35 are respectively connected in parallel to the transistors 31, 34. The intermediate point potential is detected while the transistors 31, 34 are being turned off to determine in advance the occurrence of short-circuits of the lines L1 and L2. If no short-circuits have occurred, the diagnosis is then carried out. That is, unless short-circuits occur in the lines, the intermediate point potential is an approximate V/2 (V: power source potential) since the power source potential V is divided by the resistors 32, 35.
To the contrary, if the line L2 is short-circuited to ground, the intermediate point potential is an approximate ground potential. Also, if the line L1 is short-circuited to the power supply line, the intermediate point potential is an approximate power source potential. Therefore, the short-circuits of the lines L1, L2 can be detected based on the intermediate point potential.
The intermediate point potential is, however, detected on the basis of either instantaneously sampled voltages or an average voltage at the intermediate point M. Therefore, in a case where the line L1 or the line L2 instantaneously and repeatedly makes contact with the power supply line or ground, such a short-circuit of the line L1 or the line L2 cannot be detected. For this reason, while the diagnosis is carried out and the transistors 31, 34 are alternately turned on, if the line L1 makes contact with the power supply line, or if the line L2 makes contact with ground, the large current instantaneously flows through the squib 33, thereby causing the squib 33 to ignite erroneously.